The present invention relates to producing thin metal strip by a hot rolling process. More specifically, the present invention is appropriate for producing thin stainless steel strip in a hot rolling process. Since 1950, the production of stainless steel in the western world has been doubling approximately every twenty years. About fifty percent of the total stainless steel production is made up of austenite cold strip. The majority of the austenite cold strip produced is stainless steel 304 (AISI 304). Furthermore, in terms of finished product thickness, the majority of finished product today has a strip thickness predominately in the range of 0.7 to 2.5 mm (millimeters). Based on these figures, there is a need for efficiently producing a stainless steel metal strip, specifically austenite metal strip, having a finished product thickness of about 0.7 to 2.5 mm. The present invention relates to an apparatus and method for producing such a product.
Rolling processes for carbon steel and stainless steels differ because of the differences in mechanical behavior between carbon steels and stainless steels. Stainless steels generally have a lower thermal conductivity at temperatures below about 815.degree. C. than carbon and low-alloy steels. Therefore, heating stainless steels below 815.degree. C. must be done carefully or surface burning will result. Above 815.degree. C. however, stainless steels can be heated the same as carbon steels. For most of the stainless steel grades, the temperature ranges for optimum hot-working characteristics are narrower than those for the carbon steels. Therefore, a close temperature control may be necessary when hot working stainless steels.
Ferritic stainless steels, the iron-chromium stainless steels, are typically very soft when hot, and thus they are easily marked by guides or rolls. Additionally, ferritic stainless steels spread considerably during hot rolling. Over-heating these stainless steels can cause excessive metal grain growth, which can make the materials susceptible to tears and cracks.
Austenitic stainless steels, the iron-chromium-nickel stainless steels, are typically stronger than ferritic stainless steels at rolling temperature and thus require more power for deformation. Finishing temperatures which are too low are not practical for austenitic stainless steels because of the power required for deformation. Since austenitic stainless steels are stronger, the amount of reduction per rolling pass is smaller for these stainless steel grades. These steel grades tend to spread less than do ordinary steels.
The temperature of working stainless steels is very important to the finished product. For example, ferritic stainless steels are characterized by two temperature dependent phenomenon that are important in hot rolling. The first of these phenomenon is called roping or ridging. This name signifies the ridges or surface irregularities that form as the result of working ferritic stainless steels. The surface ridges are in the direction of the final cold rolling of the product. It is known that ridging is caused by development of certain textures in the material, following the cold-reduction and annealing operations. Ridging can be reduced by employing high temperatures, for example 870.degree. C. or higher, when working the metal.
The second phenomenon of ferritic stainless steels is the 475.degree. C. embrittlement phenomenon which is a precipitation hardening phenomenon occurring when the ferritic stainless steels are heated in a range of about 370.degree. C. to 540.degree. C. This precipitation hardening can reduce the ductility and toughness of the material. In processing ferritic stainless steels into thin strip by the hot rolling process, it is typically desired to work the material at a temperature above the range of 370.degree. C. to 540.degree. C. in order to avoid the embrittlement phenomenon.
Austenitic stainless steels also have temperature dependent working properties. The temperature of working the austenitic stainless steels will impart certain properties to the hot rolled product. Austenitic stainless steels however, tend to be more stable than ferritic stainless steels during the hot rolling process, in as much as there is no precise embrittlement and ridging temperatures. Nonetheless, at elevated temperatures austenitic steels may be worked into a tough and ductile finished product.
The present invention is an improvement over current hot rolling processes for producing thin strip finished product. The current processes are deficient in that thin metal strip of 0.4 to 1.2 mm cannot be produced with the desired metallurgical characteristics. For example, while U.S. Pat. No. 4,580,428 (1986) discloses a hot rolling mill with a roughing stand and a finishing stand having different sized work rolls. This tandem arrangement of mill stands is not designed for independent temperature controlled roughing and finishing. The roughing stand and the finishing stand are adjacent to each other and are operated in tandem which limits the type of finished product that can be produced from this mill.
Another arrangement for a hot rolling process which has deficiencies for producing a large variety of the possible thin metal strip products is disclosed in U.S. Pat. No. 5,329,688 (1994). This process hot rolls a cast slab at a temperature above 1100.degree. C., followed by a warm semi-finishing rolling of the chilled strip in the temperature range of 250 to 260.degree. C. followed by final cold finishing rolling below 250.degree. C. This type of process, having a series of different temperature rollings, is not desirable for a variety of stainless steels.
Another example of a prior art process for producing thin metal strip by the hot rolling process is disclosed in U.S. Pat. No. 5,689,991 (1997). This process hot rolls thin gauge by using a reversing hot strip mill in combination with a tandem hot strip mill. Again, this arrangement cannot produce the desired thin strip from 0.4 to 1.2 mm in an independently controlled temperature hot rolling process.
The present invention overcomes the deficiencies of the prior art for producing thin metal strip by the hot rolling process.